Estudo acústico de /a/ acentuado na fala bracarense: potenciais aplicações

Dissertação de mestrado em Ciências da Linguagem

Different colors of light lead to different adaptation and activation as determined by high-density EEG.

Light adaptation is crucial for coping with the varying levels of ambient light. Using high-density electroencephalography (EEG), we investigated how adaptation to light of different colors affects brain responsiveness. In a within-subject design, sixteen young participants were adapted first to dim white light and then to blue, green, red, or white bright light (one color per session in a randomized order). Immediately after both dim and bright light adaptation, we presented brief light pulses and recorded event-related potentials (ERPs). We analyzed ERP response strengths and brain topographies and determined the underlying sources using electrical source imaging. Between 150 and 261ms after stimulus onset, the global field power (GFP) was higher after dim than bright light adaptation. This effect was most pronounced with red light and localized in the frontal lobe, the fusiform gyrus, the occipital lobe and the cerebellum. After bright light adaptation, within the first 100ms after light onset, stronger responses were found than after dim light adaptation for all colors except for red light. Differences between conditions were localized in the frontal lobe, the cingulate gyrus, and the cerebellum. These results indicate that very short-term EEG brain responses are influenced by prior light adaptation and the spectral quality of the light stimulus. We show that the early EEG responses are differently affected by adaptation to different colors of light which may contribute to known differences in performance and reaction times in cognitive tests.

Porous silicon optical filters in gas sensing applications

In this thesis, the gas sensing properties of porous silicon-based thin-film optical filters are explored. The effects of surface chemistry on the adsorption and desorption of various gases are studied in detail. Special emphasis is placed on investigating thermal carbonization as a stabilization method for optical sensing applications. Moreover, the possibility of utilizing the increased electrical conductivity of thermally carbonized porous silicon for implementing a multiparametric gas sensor, which would enable simultaneous monitoring of electrical and optical parameters, is investigated. In addition, different porous silicon-based optical filter-structures are prepared, and their properties in sensing applications are evaluated and compared. First and foremost, thermal carbonization is established as a viable method to stabilize porous silicon optical filters for chemical sensing applications. Furthermore, a multiparametric sensor, which can be used for increasing selectivity in gas sensing, is also demonstrated. Methods to improve spectral quality in multistopband mesoporous silicon rugate filters are studied, and structural effects to gas sorption kinetics are evaluated. Finally, the stability of thermally carbonized optical filters in basic environments is found to be superior in comparison to other surface chemistries currently available for porous silicon. The results presented in this thesis are of particular interest for developing novel reliable sensing systems based on porous silicon, e.g., label-free optical biosensors.

Characterization of Novel Fabry Pérot Filter Arrays for Nanospectrometers in Medical Applications

Optische Spektroskopie ist eine sehr wichtige Messtechnik mit einem hohen Potential für zahlreiche Anwendungen in der Industrie und Wissenschaft. Kostengünstige und miniaturisierte Spektrometer z.B. werden besonders für moderne Sensorsysteme “smart personal environments” benötigt, die vor allem in der Energietechnik, Messtechnik, Sicherheitstechnik (safety and security), IT und Medizintechnik verwendet werden. Unter allen miniaturisierten Spektrometern ist eines der attraktivsten Miniaturisierungsverfahren das Fabry Pérot Filter. Bei diesem Verfahren kann die Kombination von einem Fabry Pérot (FP) Filterarray und einem Detektorarray als Mikrospektrometer funktionieren. Jeder Detektor entspricht einem einzelnen Filter, um ein sehr schmales Band von Wellenlängen, die durch das Filter durchgelassen werden, zu detektieren. Ein Array von FP-Filter wird eingesetzt, bei dem jeder Filter eine unterschiedliche spektrale Filterlinie auswählt. Die spektrale Position jedes Bandes der Wellenlänge wird durch die einzelnen Kavitätshöhe des Filters definiert. Die Arrays wurden mit Filtergrößen, die nur durch die Array-Dimension der einzelnen Detektoren begrenzt werden, entwickelt. Allerdings erfordern die bestehenden Fabry Pérot Filter-Mikrospektrometer komplizierte Fertigungsschritte für die Strukturierung der 3D-Filter-Kavitäten mit unterschiedlichen Höhen, die nicht kosteneffizient für eine industrielle Fertigung sind. Um die Kosten bei Aufrechterhaltung der herausragenden Vorteile der FP-Filter-Struktur zu reduzieren, wird eine neue Methode zur Herstellung der miniaturisierten FP-Filtern mittels NanoImprint Technologie entwickelt und präsentiert. In diesem Fall werden die mehreren Kavitäten-Herstellungsschritte durch einen einzigen Schritt ersetzt, die hohe vertikale Auflösung der 3D NanoImprint Technologie verwendet. Seit dem die NanoImprint Technologie verwendet wird, wird das auf FP Filters basierende miniaturisierte Spectrometer nanospectrometer genannt. Ein statischer Nano-Spektrometer besteht aus einem statischen FP-Filterarray auf einem Detektorarray (siehe Abb. 1). Jeder FP-Filter im Array besteht aus dem unteren Distributed Bragg Reflector (DBR), einer Resonanz-Kavität und einen oberen DBR. Der obere und untere DBR sind identisch und bestehen aus periodisch abwechselnden dünnen dielektrischen Schichten von Materialien mit hohem und niedrigem Brechungsindex. Die optischen Schichten jeder dielektrischen Dünnfilmschicht, die in dem DBR enthalten sind, entsprechen einen Viertel der Design-Wellenlänge. Jeder FP-Filter wird einer definierten Fläche des Detektorarrays zugeordnet. Dieser Bereich kann aus einzelnen Detektorelementen oder deren Gruppen enthalten. Daher werden die Seitenkanal-Geometrien der Kavität aufgebaut, die dem Detektor entsprechen. Die seitlichen und vertikalen Dimensionen der Kavität werden genau durch 3D NanoImprint Technologie aufgebaut. Die Kavitäten haben Unterschiede von wenigem Nanometer in der vertikalen Richtung. Die Präzision der Kavität in der vertikalen Richtung ist ein wichtiger Faktor, der die Genauigkeit der spektralen Position und Durchlässigkeit des Filters Transmissionslinie beeinflusst.

The impact of ice crystals on radiative forcing and remote sensing of arctic boundary-layer mixed-phase clouds

This PhD thesis is embedded into the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) and investigates the radiative transfer through Arctic boundary-layer mixed-phase (ABM) clouds. For this purpose airborne spectral solar radiation measurements and simulations of the solar and thermal infrared radiative transfer have been performed. This work reports on measurements with the Spectral Modular Airborne Radiation measurement sysTem (SMART-Albedometer) conducted in the framework of ASTAR in April 2007 close to Svalbard. For ASTAR the SMART-Albedometer was extended to measure spectral radiance. The development and calibration of the radiance measurements are described in this work. In combination with in situ measurements of cloud particle properties provided by the Laboratoire de M¶et¶eorologie Physique (LaMP) and simultaneous airborne lidar measurements by the Alfred Wegener Institute for Polar and Marine Research (AWI) ABM clouds were sampled. The SMART-Albedometer measurements were used to retrieve the cloud thermodynamic phase by three different approaches. A comparison of these results with the in situ and lidar measurements is presented in two case studies. Beside the dominating mixed-phase clouds pure ice clouds were found in cloud gaps and at the edge of a large cloud field. Furthermore the vertical distribution of ice crystals within ABM clouds was investigated. It was found that ice crystals at cloud top are necessary to describe the observed SMART-Albedometer measurements. The impact of ice crystals on the radiative forcing of ABM clouds is in vestigated by extensive radiative transfer simulations. The solar and net radiative forcing was found to depend on the ice crystal size, shape and the mixing ratio of ice crystals and liquid water droplets.

Influence of ice crystal habit and cirrus spatial inhomogeneities on the retrieval of cirrus optical thickness and effective radius

Diese Dissertation untersucht den Einfluss von Eiskristallform und räumlicher Inhomogenität von Zirren auf das Retrieval von optischer Wolkendicke und effektivem Eispartikelradius. Zu diesem Zweck werden flugzeuggetragene spektrale Messungen solarer Strahlung sowie solare und langwellige Strahlungstransfersimulationen durchgeführt. Flugzeuggetragene spektrale aufwärtsgerichtete Radianzen (Strahldichten) sind mit dem SMART-Albedometer (Spectral Modular Airborne Radiation measurement sysTem) während des CIRCLE-2 (CIRrus CLoud Experiment-2) Feldexperiments im Mai 2007 gemessen worden. Basierend auf diesen Radianzdaten werden mittels eines Wolkenretrievalalgorithmus optische Wolkendicken und effektive Eispartikelradien anhand von eindimensionalen Strahlungstransferrechnungen bestimmt. Die Auswirkung der Annahme unterschiedlicher Eiskristallformen auf die retrievten Parameter wird durch Variation der Einfachstreueigenschaften der Eispartikel untersucht. Darüber hinaus wird mittels Strahlungstransferrechnungen auch der Einfluss der Eiskristallform auf den Strahlungsantrieb von Eiswolken ermittelt. Die Frage nach dem relativen Einfluss von räumlicher Wolkeninhomogenität und Eiskristallform wird anhand von dreidimensionalen und independent pixel approximation (IPA) Strahlungssimulationen untersucht. Die Analyse basiert auf einer Modelleiswolke, die aus Daten des NASA (National Aeronautics and Space Administration) TC4 (Tropical Composition, Cloud, and Climate Coupling) Feldexperiments im Sommer 2007 in Costa Rica erzeugt wurde. Lokal gesehen können beide Effekte - Eiskristallform und räumliche Eiswolkeninhomogenität - die gleiche Grössenordnung haben und zu einer Unter- bzw. Überschätzung der retrievten Parameter um 40 – 60% führen. Gemittelt über die ganze Wolke ist jedoch der Einfluss der Eiskristallform viel bedeutender als der von räumlichen Inhomogenitäten.

Zoë Wicomb's Ghosts. Uncanny Translocations in David's Story and The One That Got Away

This essay analyses Zoë Wicomb's novel David's Story and her latest collection of short stories, The One That Got Away, through the lense of cosmopolitanism and Jacques Derrida's concept of ‘hauntology’. Wicomb is a cosmopolitan author in a very precise sense: an author who embeds locally specific stories in a complex intertextual, historical and transnational web of cross-references. As settings, characters and objects move between Scotland and South Africa, it appears that the histories of these countries are mutually haunted by each other. Uncanny encounters with the past, and with memorials and art objects that take on a spectral quality, evoke an increasing sense of disorientation on the part of protagonists and readers alike. Assumptions about place, history and identity are thus constantly undermined and reconfigured.

Magnetic resonance spectroscopy investigations of functionally defined language areas in schizophrenia patients with and without auditory hallucinations

Background: Cerebral dysfunction occurring in mental disorders can show metabolic disturbances which are limited to circumscribed brain areas. Auditory hallucinations have been shown to be related to defined cortical areas linked to specific language functions. Here, we investigated if the study of metabolic changes in auditory hallucinations requires a functional rather than an anatomical definition of their location and size to allow a reliable investigation by magnetic resonance spectroscopy (MRS). Methods: Schizophrenia patients with (AH; n = 12) and without hallucinations (NH; n = 8) and healthy controls (HC; n = 11) underwent a verbal fluency task in functional MRI (fMRI) to functionally define Broca's and Wernicke's areas. Left and right Heschl's gyri were defined anatomically. Results: The mean distances in native space between the fMRI-defined regions and a corresponding anatomically defined area were 12.4 ± 6.1 mm (range: 2.7–36.1 mm) for Broca's area and 16.8 ± 6.2 mm (range: 4.5–26.4 mm) for Wernicke's area, respectively. Hence, the spatial variance was of similar extent as the size of the investigated regions. Splitting the investigations into a single voxel examination in the frontal brain and a spectroscopic imaging part for the more homogeneous field areas led to good spectral quality for almost all spectra. In Broca's area, there was a significant group effect (p = 0.03) with lower levels of N-acetyl-aspartate (NAA) in NH compared to HC (p = 0.02). There were positive associations of NAA levels in the left Heschl's gyrus with total (p = 0.03) and negative (p = 0.006) PANSS scores. In Broca's area, there was a negative association of myo-inositol levels with total PANSS scores (p = 0.008). Conclusion: This study supports the neurodegenerative hypothesis of schizophrenia only in a frontal region whereas the results obtained from temporal regions are in contrast to the majority of previous studies. Future research should test the hypothesis raised by this study that a functional definition of language regions is needed if neurochemical imbalances are expected to be restricted to functional foci.

The Value of Adding Optics to Ecosystem Models: A Case Study

Many ecosystem models have been developed to study the ocean's biogeochemical properties, but most of these models use simple formulations to describe light penetration and spectral quality. Here, an optical model is coupled with a previously published ecosystem model that explicitly represents two phytoplankton (picoplankton and diatoms) and two zooplankton functional groups, as well as multiple nutrients and detritus. Surface ocean color fields and subsurface light fields are calculated by coupling the ecosystem model with an optical model that relates biogeochemical standing stocks with inherent optical properties (absorption, scattering); this provides input to a commercially available radiative transfer model (Ecolight). We apply this bio-optical model to the equatorial Pacific upwelling region, and find the model to be capable of reproducing many measured optical properties and key biogeochemical processes in this region. Our model results suggest that non-algal particles largely contribute to the total scattering or attenuation (> 50% at 660 nm) but have a much smaller contribution to particulate absorption (< 20% at 440 nm), while picoplankton dominate the total phytoplankton absorption (> 95% at 440 nm). These results are consistent with the field observations. In order to achieve such good agreement between data and model results, however, key model parameters, for which no field data are available, have to be constrained. Sensitivity analysis of the model results to optical parameters reveals a significant role played by colored dissolved organic matter through its influence on the quantity and quality of the ambient light. Coupling explicit optics to an ecosystem model provides advantages in generating: (1) a more accurate subsurface light-field, which is important for light sensitive biogeochemical processes such as photosynthesis and photo-oxidation, (2) additional constraints on model parameters that help to reduce uncertainties in ecosystem model simulations, and (3) model output which is comparable to basic remotely-sensed properties. In addition, the coupling of biogeochemical models and optics paves the road for future assimilation of ocean color and in-situ measured optical properties into the models.

Transformada Wavelet Compleja de Doble Árbol (DT-CWT) y su aplicación en Teledetección. Nuevos modelos y aplicaciones

La teledetección o percepción remota (remote sensing) es la ciencia que abarca la obtención de información (espectral, espacial, temporal) sobre un objeto, área o fenómeno a través del análisis de datos adquiridos por un dispositivo que no está en contacto con el elemento estudiado. Los datos obtenidos a partir de la teledetección para la observación de la superficie terrestre comúnmente son imágenes, que se caracterizan por contar con un sinnúmero de aplicaciones que están en continua evolución, por lo cual para solventar los constantes requerimientos de nuevas aplicaciones a menudo se proponen nuevos algoritmos que mejoran o facilitan algún proceso en particular. Para el desarrollo de dichos algoritmos, es preciso hacer uso de métodos matemáticos que permitan la manipulación de la información con algún fin específico. Dentro de estos métodos, el análisis multi-resolución se caracteriza por permitir analizar una señal en diferentes escalas, lo que facilita trabajar con datos que puedan tener resoluciones diferentes, tal es el caso de las imágenes obtenidas mediante teledetección. Una de las alternativas para la implementación de análisis multi-resolución es la Transformada Wavelet Compleja de Doble Árbol (DT-CWT). Esta transformada se implementa a partir de dos filtros reales y se caracteriza por presentar invariancia a traslaciones, precio a pagar por su característica de no ser críticamente muestreada. A partir de las características de la DT-CWT se propone su uso en el diseño de algoritmos de procesamiento de imagen, particularmente imágenes de teledetección. Estos nuevos algoritmos de procesamiento digital de imágenes de teledetección corresponden particularmente a fusión y detección de cambios. En este contexto esta tesis presenta tres algoritmos principales aplicados a fusión, evaluación de fusión y detección de cambios en imágenes. Para el caso de fusión de imágenes, se presenta un esquema general que puede ser utilizado con cualquier algoritmo de análisis multi-resolución; este algoritmo parte de la implementación mediante DT-CWT para luego extenderlo a un método alternativo, el filtro bilateral. En cualquiera de los dos casos la metodología implica que la inyección de componentes pueda realizarse mediante diferentes alternativas. En el caso del algoritmo de evaluación de fusión se presenta un nuevo esquema que hace uso de procesos de clasificación, lo que permite evaluar los resultados del proceso de fusión de forma individual para cada tipo de cobertura de uso de suelo que se defina en el proceso de evaluación. Esta metodología permite complementar los procesos de evaluación tradicionales y puede facilitar el análisis del impacto de la fusión sobre determinadas clases de suelo. Finalmente, los algoritmos de detección de cambios propuestos abarcan dos enfoques. El primero está orientado a la obtención de mapas de sequía en datos multi-temporales a partir de índices espectrales. El segundo enfoque propone la utilización de un índice global de calidad espectral como filtro espacial. La utilización de dicho filtro facilita la comparación espectral global entre dos imágenes, esto unido a la utilización de umbrales, conlleva a la obtención de imágenes diferencia que contienen la información de cambio. ABSTRACT Remote sensing is a science relates to information gathering (spectral, spatial, temporal) about an object, area or phenomenon, through the analysis of data acquired by a device that is not in contact with the studied item. In general, data obtained from remote sensing to observe the earth’s surface are images, which are characterized by having a number of applications that are constantly evolving. Therefore, to solve the constant requirements of applications, new algorithms are proposed to improve or facilitate a particular process. With the purpose of developing these algorithms, each application needs mathematical methods, such as the multiresolution analysis which allows to analyze a signal at different scales. One of the options is the Dual Tree Complex Wavelet Transform (DT-CWT) which is implemented from two real filters and is characterized by invariance to translations. Among the advantages of this transform is its successful application in image fusion and change detection areas. In this regard, this thesis presents three algorithms applied to image fusion, assessment for image fusion and change detection in multitemporal images. For image fusion, it is presented a general outline that can be used with any multiresolution analysis technique; this algorithm is proposed at first with DT-CWT and then extends to an alternative method, the bilateral filter. In either case the method involves injection of components by various means. For fusion assessment, the proposal is focused on a scheme that uses classification processes, which allows evaluating merger results individually for each type of land use coverage that is defined in evaluation process. This methodology allows complementing traditional assessment processes and can facilitate impact analysis of the merger on certain kinds of soil. Finally, two approaches of change detection algorithms are included. The first is aimed at obtaining drought maps in multitemporal data from spectral indices. The second one takes a global index of spectral quality as a spatial filter. The use of this filter facilitates global spectral comparison between two images and by means of thresholding, allows imaging containing change information.

Infrared microspectroscopy of inflammatory process and colon tumors.

According to the last global burden of disease published by the World Health Organization, tumors were the third leading cause of death worldwide in 2004. Among the different types of tumors, colorectal cancer ranks as the fourth most lethal. To date, tumor diagnosis is based mainly on the identification of morphological changes in tissues. Considering that these changes appears after many biochemical reactions, the development of vibrational techniques may contribute to the early detection of tumors, since they are able to detect such reactions. The present study aimed to develop a methodology based on infrared microspectroscopy to characterize colon samples, providing complementary information to the pathologist and facilitating the early diagnosis of tumors. The study groups were composed by human colon samples obtained from paraffin-embedded biopsies. The groups are divided in normal (n=20), inflammation (n=17) and tumor (n=18). Two adjacent slices were acquired from each block. The first one was subjected to chemical dewaxing and H&E staining. The infrared imaging was performed on the second slice, which was not dewaxed or stained. A computational preprocessing methodology was employed to identify the paraffin in the images and to perform spectral baseline correction. Such methodology was adapted to include two types of spectral quality control. Afterwards the preprocessing step, spectra belonging to the same image were analyzed and grouped according to their biochemical similarities. One pathologist associated each obtained group with some histological structure based on the H&E stained slice. Such analysis highlighted the biochemical differences between the three studied groups. Results showed that severe inflammation presents biochemical features similar to the tumors ones, indicating that tumors can develop from inflammatory process. A spectral database was constructed containing the biochemical information identified in the previous step. Spectra obtained from new samples were confronted with the database information, leading to their classification into one of the three groups: normal, inflammation or tumor. Internal and external validation were performed based on the classification sensitivity, specificity and accuracy. Comparison between the classification results and H&E stained sections revealed some discrepancies. Some regions histologically normal were identified as inflammation by the classification algorithm. Similarly, some regions presenting inflammatory lesions in the stained section were classified into the tumor group. Such differences were considered as misclassification, but they may actually evidence that biochemical changes are in course in the analyzed sample. In the latter case, the method developed throughout this thesis would have proved able to identify early stages of inflammatory and tumor lesions. It is necessary to perform additional experiments to elucidate this discrepancy between the classification results and the morphological features. One solution would be the use of immunohistochemistry techniques with specific markers for tumor and inflammation. Another option includes the recovering of the medical records of patients who participated in this study in order to check, in later times to the biopsy collection, whether they actually developed the lesions supposedly detected in this research.

The XMM deep survey in the CDF-S. VI. Obscured AGN selected as infrared power-law galaxies

Context. Accretion onto supermassive black holes is believed to occur mostly in obscured active galactic nuclei (AGN). Such objects are proving rather elusive in surveys of distant galaxies, including those at X-ray energies. Aims. Our main goal is to determine whether the revised IRAC criteria of Donley et al. (2012, ApJ, 748, 142; objects with an infrared (IR) power-law spectral shape), are effective at selecting X-ray type-2 AGN (i.e., absorbed N_H > 10^22 cm^-2). Methods. We present the results from the X-ray spectral analysis of 147 AGN selected by cross-correlating the highest spectral quality ultra-deep XMM-Newton and the Spitzer/IRAC catalogues in the Chandra Deep Field South. Consequently it is biased towards sources with high S/N X-ray spectra. In order to measure the amount of intrinsic absorption in these sources, we adopt a simple X-ray spectral model that includes a power-law modified by intrinsic absorption at the redshift of each source and a possible soft X-ray component. Results. We find 21/147 sources to be heavily absorbed but the uncertainties in their obscuring column densities do not allow us to confirm their Compton-Thick nature without resorting to additional criteria. Although IR power-law galaxies are less numerous in our sample than IR non-power-law galaxies (60 versus 87 respectively), we find that the fraction of absorbed (N_H^intr > 10^22 cm^-2) AGN is significantly higher (at about 3 sigma level) for IR-power-law sources (similar to 2/3) than for those sources that do not meet this IR selection criteria (~1/2). This behaviour is particularly notable at low luminosities, but it appears to be present, although with a marginal significance, at all luminosities. Conclusions. We therefore conclude that the IR power-law method is efficient in finding X-ray-absorbed sources. We would then expect that the long-sought dominant population of absorbed AGN is abundant among IR power-law spectral shape sources not detected in X-rays.

Point-by-point inscription of first-order fiber Bragg grating for C-band applications

The influence of the fiber geometry on the point-by-point inscription of fiber Bragg gratings using a femtosecond laser is highlighted. Fiber Bragg gratings with high spectral quality and strong first-order Bragg resonances within the C-band are achieved by optimizing the inscription process. Large birefringence (1.2×10-4) and high degree of polarizationdependent index modulation are observed in these gratings. Potential applications of these gratings in resonators are further illustrated.

Ultrashort pulse generation from diode laser devices

This thesis presents a detailed, experiment-based study of generation of ultrashort optical pulses from diode lasers. Simple and cost-effective techniques were used to generate high power, high quality optical short pulses at various wavelength windows. The major achievements presented in the thesis is summarised as follows. High power pulses generation is one of the major topics discussed in the thesis. Although gain switching is the simplest way for ultrashort pulse generation, it proves to be quite effective to deliver high energy pulses on condition that the pumping pulses with extremely fast rising time and high enough amplitude are applied on specially designed pulse generators. In the experiment on a grating-coupled surface emitting laser (GCSEL), peak power as high as 1W was achieved even when its spectral bandwidth was controlled within 0.2nm. Another experiment shows violet picosecond pulses with peak power as high as 7W was achieved when the intensive electrical pulses were applied on optimised DC bias to pump on InGaN violet diode laser. The physical mechanism of this phenomenon, as we considered, may attributed to the self-organised quantum dots structure in the laser. Control of pulse quality, including spectral quality and temporal profile, is an important issue for high power pulse generation. The ways to control pulse quality described in the thesis are also based on simple and effective techniques. For instance, GCSEL used in our experiment has a specially designed air-grating structure for out-coupling of optical signals; hence, a tiny flat aluminium mirror was placed closed to the grating section and resulted in a wavelength tuning range over 100nm and the best side band suppression ratio of 40dB. Self-seeding, as an effective technique for spectral control of pulsed lasers, was demonstrated for the first time in a violet diode laser. In addition, control of temporal profile of the pulse is demonstrated in an overdriven DFB laser. Wavelength tuneable fibre Bragg gratings were used to tailor the huge energy tail of the high power pulse. The whole system was compact and robust. The ultimate purpose of our study is to design a new family of compact ultrafast diode lasers. Some practical ideas of laser design based on gain-switched and Q-switched devices are also provided in the end.

Characterizing femtosecond laser inscribed Bragg grating spectra

We present numerical modeling based on a combination of the Bidirectional Beam Propagation Method and Finite Element Method that completely describes the wavelength spectra of point by point femtosecond laser inscribed fiber Bragg gratings, showing excellent agreement with experiment. We have investigated the dependence of different spectral parameters such as insertion loss, all dominant cladding and ghost modes and their shape relative to the position of the fiber Bragg grating in the core of the fiber. Our model is validated by comparing model predictions with experimental data and allows for predictive modeling of the gratings. We expand our analysis to more complicated structures, where we introduce symmetry breaking; this highlights the importance of centered gratings and how maintaining symmetry contributes to the overall spectral quality of the inscribed Bragg gratings. Finally, the numerical modeling is applied to superstructure gratings and a comparison with experimental results reveals a capability for dealing with complex grating structures that can be designed with particular wavelength characteristics. (C) 2010 Optical Society of America